In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a vascular passageway and which are subsequently expandable into contact with the walls of the passageway. These devices include, among others, blood clot filters which expand and are held in position by engagement with the inner wall of a vein, such as the vena cava. Vena cava filters are known in the art as described, for example, in U.S. Pat. Nos. 4,425,908, 5,669,933 and 5,836,968, and European Patent Office publication 0 188 927 A2, each of which are incorporated by reference herein in their entirety. Such filters may include structure to anchor the filter in place within the vena cava, such as elongated diverging anchor members with hooked ends that penetrate the vessel wall and positively prevent longitudinal migration within the vessel. Such filters also may include locator structures to position the filter within the blood vessel, particularly with respect to the centerline of the vessel. Such locator structures may consist of a number of members which press against the walls of the vessel with approximately equal force, thus causing the center of the filter to move to the centerline of the vessel. A filter including anchor members having hooked ends and locator members is disclosed in U.S. Pat. No. 6,258,026, which is incorporated by reference herein in its entirety.
Referring to FIG. 20, a known expanded blood clot filter 10 is illustrated which is made from sets of elongated metal wires. The wires are held together at one end by a hub 12 where they may be plasma welded together and to the hub or otherwise joined. In one material phase, the wires, which are made of a shape memory material, can be straightened and held in a straight form that can pass through a length of fine plastic tubing with an internal diameter of approximately 2 mm (No. 7 French diameter catheter). At a defined temperature, the filter 10 recovers a preformed filtering shape as illustrated by FIG. 20. Similarly, wires of spring metal can be straightened and compressed within a catheter or tube and will diverge into the filter shape of FIG. 20 when the tube is removed. The blood filter 10 is typically delivered into a subject's blood vessel by being pushed out of a catheter positioned within the vein.
In its normal expanded configuration or preformed filtering shape, illustrated in FIG. 20, filter 10 is a double filter, having a first forwardly disposed filter basket 14 at the forward or leading end of the filter and a second forwardly disposed filter basket 16. The two filter baskets provide peripheral portions which can both engage the inner wall of a blood vessel at two longitudinally spaced locations, and the two filter baskets are generally symmetrical about a longitudinal axis passing through the hub 12.
The first filter basket 14 is formed by the anchor members 30, with up to twelve circumferentially spaced curved or linear wires forming the anchor members, which extend away from hub 12 and away from the longitudinal axis of the filter 10 and end in hooks 40, such as those illustrated in FIG. 21. The outwardly oriented hooks 40 generally lie on a circle at the maximum divergence of the anchor members 30. Six anchor members 30 are shown in FIG. 20. The anchor members may be of equal length, but normally the lengths differ so that the hooks 40 will fit within a catheter without becoming interconnected. The anchor members 30 may be much longer than the locators 20. In the expanded configuration shown in FIG. 20, the anchor members 30 are at a slight angle to the vessel wall, preferably within a range of from ten to forty-five degrees, while the hooks 40 penetrate the vessel wall to secure the filter against movement.
The second filter basket 16 is formed by locators 20 that extend angularly with respect to the longitudinal axis, outwardly and then downwardly from the hub 12 toward the forward end of the filter 10. As is shown in FIG. 20, each locator 20 may have a first locator section 21 which extends angularly out from the hub 12 to a shoulder 22, and an outer locator section 24 that extends angularly from the shoulder toward the forward end of the filter. Typically, there are six locators 20 of equal length extending radially outward from the hub 12 and circumferentially spaced, such as for example by sixty degrees of arc.
The anchors 30 may be radially offset relative to the locators 20 and may be positioned halfway between the locators 20 and also may be circumferentially spaced by sixty degrees of arc as shown in FIG. 22. Thus the combined filter baskets 14 and 16 can provide a wire positioned every thirty degrees of arc at the maximum divergence of the filter sections. With reference to the direction of blood flow, filter basket 14 forms a first concave opening toward the leading end of filter 10 and filter basket 16 forms a second concave opening toward the leading end of filter 10 downstream of filter basket 14.
For a filter to properly deploy the first and second filter baskets within the blood vessel, it is preferred that the filter hub 12 be positioned substantially along the centerline of the vessel. This centering function is performed by the locator members which have outer locator sections 24 that lie on a circle at their maximum divergence and engage the wall 17 of a vessel (preferably at an angle within a range of from ten to forty-five degrees) to center the hub 12 within the vessel. This is illustrated in FIG. 22. When positioned within a blood vessel, the locator members apply radial pressure to the walls, thereby pushing the filter hub 2 toward the vessel centerline. When a filter 10, such as that illustrated in FIG. 20, is ejected from an insertion catheter hub-end first, the locator members 20 will deploy first. Since the as-deployed radial separation between locator tips is larger than the diameter of the blood vessel, the locator tips contact and push against the walls of the blood vessel, as illustrated in FIG. 22, thereby centering the filter in the blood vessel before the anchor members deploy from the delivery catheter. In addition to serving as filter elements for catching blood clots, this centering function is an important function of the locator members.
Blood filters which use locator members as described above suffer from the disadvantage that the locator members do not contribute to anchoring the filter longitudinally in the blood vessel. This is because the locator members must be able to move with respect to the vessel wall while the filter centers in the blood vessel. If the locator members included hooks, they could hook into the vessel walls before the filter centers, leaving the filter in a cocked position. Accordingly, there is a need for a filter that includes hooks on locator members which hook into the vessel wall after the filter is centered in the blood vessel.